Scanning electron microscopes and generally instruments employing an electron beam (probe) operate in vacuum (pressure less than about 0.0001 mbar) and the specimens examined by such instruments are also placed in vacuum. Scanning a sample within a vacuum presents many problems. Many biological specimens cannot survive in vacuum. Wet specimens can experience evaporation of their fluid content before an accurate image can be obtained. Nonconducting samples can accumulate a surface charge which obscures the details of the sample's surface and lowers the resolution of the image obtained.
An environmental scanning electron microscope (ESEM) which allows the examination of specimens in a gaseous environment is described in U.S. Pat. No. 4,596,928. The purpose of the gas in the '928 patent was to act as a conditioning medium in order to maintain a specimen in a liquid, set or natural state. However, the predominant detection mode in the ESEM has utilized various scintillator detectors to detect backscattered electrons. Additionally, an ESEM detection system has been described wherein the ionization of the gaseous environment is used as the detection means for all ionizing signals (Danilatos, Micron. Microsc. Acta 14:307-318, 1983).